In diagnosis using a nuclear magnetic resonance, the required accuracy in magnetic intensity of a magnet system is such that variation of one millionth in magnetic intensity is considered to be a problem since the magnetic intensity corresponds to a diagnosis location. There are three types of magnetic fields in MRI apparatuses. That is:
(1) A magnetic field that is a constant in time base and uniform in space, and has an intensity of generally more than 0.1 to several teslas and a variation range of about several ppm within a space for imaging (a space of a sphere or an ellipsoid with a diameter of 30 to 40 cm);
(2) A magnetic field varying with a time constant of about one second or shorter and inclined in a space; and
(3) A magnetic field caused by a high frequency wave having a frequency (several MHz or higher) corresponding to the nuclear magnetic resonance.
The present invention focuses on a static magnetic field of (1). This magnetic field is required to have constant intensity in time base and spatially have homogeneity in the intensity with extremely high accuracy in a region where tomographic imaging of a human body is performed, especially, in a case of a magnetic resonance imaging apparatus.
The high accuracy mentioned here indicates the accuracy of a residual magnetic field with an order of one millionth, such as ±1.5 ppm, in an imaging space FOV (Field of View) with a diameter of, for example, 40 cm. A magnetic field distribution of which homogeneity is required to be extremely high is realized by adjusting a magnetic field after production and excitation of a magnet with high accuracy.
Generally, the magnitude of an error magnetic field due to a production error is 1000 times or more greater than the magnitude of the permissible error magnetic field demanded for a uniform magnetic field. Therefore, magnetic field adjustment (shimming) required when the apparatus is installed after production requires a magnetic field adjustment apparatus and a method with extremely high accuracy since an error magnetic field is reduced from hundreds ppm to several ppm.
As a method of the related art, PTL 1 discloses that arrangement of magnetic moment using singular value decomposition is computed, and shimming is performed by using a result thereof. In PTL 1, a distribution of magnetic moment or an iron piece volume is computed by using truncated singular value decomposition and a current potential, and iron piece arrangement magnetic field measurement work is performed on the basis of a result thereof.
PTL 1 discloses an example in which magnetic bodies for shimming are continuously disposed. FIGS. 2A and 2B illustrate a system of shimming computation and a computation result disclosed in PTL 1. In a shimming method of the related art in FIGS. 2A and 2B, FIG. 2A is a figure illustrating a shimming computation system, and FIG. 2B is a figure for explaining an eigenmode intensity and an eigenmode selected for shimming. According to PTL 1, a shimming iron volume is calculated in numerical values for each region on a continuous plane according to a contour line, but the shimming iron volume is not a discrete volume in which a standardized unit iron volume is reflected.
However, shimming is often to be performed after a volume and a position of iron to be disposed are standardized, that is, a volume and a position of iron to be disposed are discretized from the viewpoint of productivity and workability, depending on models. The term “discrete” mentioned here indicates two meanings such as spatially being discretized, and the minimum unit being present even in a shimming iron volume.
As an approach regarding such discrete arrangement, for example, there is PTL 2.